Electric utility grids have traditionally been arranged so that power flows radially outward from a centrally located power plant to multiple points of usage. The increasing use of renewable power sources, however, has introduced distributed generation (DG) capacity to power grids. DG power sources may be located anywhere on the grid, typically close to a local power load. Examples of DG power sources include photovoltaic (PV) panels and wind turbines which are scattered at customer locations throughout the grid. At certain times, all of the power from these sources may be consumed by local building loads, while at other times, excess power is fed back into the grid. Thus, utility grids have become complex, interconnected structures with power flowing in multiple directions depending on the availability of power from multiple sources and demand from multiple loads at any specific time.
Distributed power sources must include synchronization functionality to enable power from the distributed source to be injected into the grid in phase with the power already flowing in the grid. Most commonly, this functionality is implemented with a phase-locked loop (PLL) which generates a local sine wave reference having the same phase and frequency as the power grid. This reference is then used to inject current in phase with the grid.
“Islanding” is a condition in which a portion of the utility grid containing power generation capacity and load becomes isolated from the remainder of the grid, but continues to operate independently because the PLL or other synchronization functionality continues to provide a reference for the power flowing in the isolated portion of the grid. Islanding is problematic, however, because it typically degrades the quality of power flowing in the isolated portion of the grid, creates unsafe conditions for utility workers, causes mismatches when the isolated portion of the grid is eventually reconnected to the main grid, and may cause numerous other problems including mismatches between power generation capacity and demand. Thus, if an islanding condition is detected, the local power generation capacity should be disconnected. This is referred to as anti-islanding (AI) protection, and the detection of islanding conditions is an ongoing challenge.
Numerous islanding detection techniques have been developed. Some of the most effective techniques involve the use of positive feedback in the distributed generation control system. A common method is to place a narrow-band low-pass filter and amplification in the grid voltage measurement and current injection feedback loop, with sufficient gain to provide a low frequency (<grid frequency) instability and oscillation that builds up when the grid is disconnected, and dampens down when the grid is connected. Noise seeds the oscillation growth.
FIG. 1 illustrates a prior art distributed generation control system having a positive feedback anti-islanding feature. An inverter bridge 10 converts DC power from a DC power source 12 to AC power which is delivered to a local load 14 and utility grid 16 at a point of common coupling (PCC) 18. A disconnect switch 20 or utility circuit breaker/recloser may isolate the local load 14 from the utility grid 16 in response to one or more fault conditions.
The normal negative feedback loop includes a phase-locked loop 22 which generates frequency (ω) and phase (θ) reference signals in response to the output voltage va from the inverter at the point of common coupling. An error generator 24 generates a feedback signal ERROR in response to a sample of the output current ia and a current reference signal iREF. A pulse width modulation (PWM) circuit 26 generates switching signals to control the inverter bridge in response to the signals from the error generator and PLL.
The positive feedback portion of the control system includes a low-pass or band-pass filter 28, an amplifier 30 that determines the loop gain, and a summing circuit 32. In the absence of the positive feedback loop, the PLL and negative feedback loop through error generator 24 would cause the inverter to continue operating long after the inverter and local load are isolated from the utility grid (islanded). The phase and/or frequency of the PLL, however, would slowly drift until a problematic condition develops.
The positive portion of the feedback loop introduces a small amount of positive feedback through summing circuit 32. When the inverter and local load are connected to the utility grid, the low impedance of the grid overcomes the effect of the positive feedback, and the output of the inverter remains stable. When the inverter and local load are disconnected from the utility grid, however, the positive feedback loop causes one or more parameters of the inverter output to grow, decay or oscillate until it trips a protection feature such as over/under voltage protection (OVP/UVP), over/under current protection (OCP/UCP), or over/under frequency protection (OFP/UFP) which is included in the control system, but not shown in FIG. 1.
Although prior art positive feedback anti-islanding techniques such as the one illustrated in FIG. 1 may provide adequate performance in some situations, they suffer from some drawbacks. For example, the presence of the filter in the positive feedback loop causes a significant amount of loop delay which slows down the response to an islanding condition. The positive feedback loop typically provides a 20 Hz oscillation in response to an islanding condition which results in a time constant that may be too long to trigger a shutdown during the time periods mandated by utility grid interconnect standards and regulations. The system of FIG. 1 may also have difficulty re-synchronizing when grid power is restored.